The purpose of this study is to evaluate the safety and efficacy of Terbogrel in patients with Primary Pulmonary Hypertension. Primary pulmonary hypertension is a condition in which the blood pressure is abnormally high in the blood vessels that go from the heart to the lungs (pulmonary arteries). Although the cause of this high blood pressure in unknown, previous studies have shown an imbalance between two compounds in the blood in patients with primary pulmonary hypertension: An increased amount of thromboxane and a decreased amount of prostacyclin. Thromboxane causes pulmonary vasoconstriction, e.g. closes the blood vessels in the lungs, which increases the blood pressure in the lungs, as opposed to prostacyclin that opens the blood vessels in the lungs, thereby decreasing the blood pressure in the lungs. Previous studies demonstrated that continuous intravenous infusion of prostacyclin in patients with primary pulmonary hypertension improved quality of life, improved survival and decreased the blood pressure of the lungs. Prostacyclin, subsequently, was approved by the Food and Drug Administration for long-term treatment of primary pulmonary hypertension. Unfortunately, chronic prostacyclin can only be given by continuous intravenous infusion, 24 hours a day. Despite therapy with chronic continuous infusion of prostacyclin substantially improving survival and quality of life, it is associated with significant risks due to the delivery system, including infections with subsequent hospitalizations. Terbogrel is a potent thromboxane receptor antagonist and thromboxane synthase inhibitor. It is available in an oral preparation, in addition to an intravenous form. Terbogrel is potentially capable of correcting the imbalance between thromboxane and prostacyclin in patients with primary pulmonary hypertension. Because of the potential for a therapeutic effect and an urgent need for an alternative effective treatment, the safety and efficacy of Terbogrel will be evaluated in patients with primary pulmonary hypertension. The study design is an international multicenter, randomized, double-blind, placebo-controlled evaluation of the safety and efficacy of Terbogrel in patients with primary pulmonary hypertension, comparing Terbogrel plus conventional therapy to conventional therapy alone in 145 patients with primary pulmonary hypertension.